1. Field of the Invention
The present invention relates to a memory composed of a storage layer for storing therein information based on the magnetization state of a ferromagnetic layer, a magnetization fixed layer of which magnetization direction is fixed and a storage element in which the magnetization direction of the storage layer is changed with application of an electric current, and particularly relates to a memory suitable for use as the application to a nonvolatile memory.
2. Description of the Related Art
As information equipment, in particular, personal small information equipment such as personal digital assistants is widespread rapidly, it is requested that devices such as memory and logic constructing personal small information equipment should become higher in performance in such a way as to become higher in integration degree, to become higher in operation speed and to become smaller in power consumption.
In particular, technologies to make semiconductor nonvolatile memories become higher in speed and larger in storage capacity become more important as complementary technologies to a magnetic hard disk which has been so far essentially difficult to be miniaturized, to become higher in speed and to become lower in power consumption due to the presence of movable parts and the like. Also, the above-mentioned technologies to realize the above-mentioned high-speed and large-capacity semiconductor nonvolatile memory become more important in order to realize new functions such as a so-called “instant on” by which an operation system can get started as the same time it is energized.
A semiconductor flash memory and a FeRAM (ferroelectric nonvolatile memory) and the like are now commercially available as the nonvolatile memory, and such nonvolatile memories are now under active research and development in order to make nonvolatile memories become higher in performance.
In recent years, as a new nonvolatile memory using as a magnetic material, a MRAM (magnetic random-access memory) using a tunnel magnetoresistive effect has been developed and advanced so far remarkably and it now receives a remarkable attention (see Cited Non-Patent References 1 and 2, for example).
This MRAM has a structure in which very small magnetic memory devices to record information are located regularly, wirings, for example, word lines and bit lines being provided to access these magnetic memory devices.
Each magnetic memory device includes a storage layer to record information as the magnetization direction of a ferromagnetic material.
Then, as the arrangement of the magnetic memory device, there is employed a structure using a so-called magnetic tunnel junction (MTJ: magnetic tunnel junction) composed of the above-mentioned storage layer, a tunnel insulating layer (non-magnetic spacer film) and a magnetization fixed layer whose magnetization direction is fixed. The magnetization direction of the magnetization fixed layer can be fixed by providing an antiferromagnetic layer, for example.
Since this structure generates a so-called tunnel magnetoresistive effect in which a resistance value relative to a tunnel electric current flowing through the tunnel insulating film changes in response to an angle formed between the magnetization direction of the storage layer and the magnetization direction of the magnetization fixed layer, it is possible to write (record) information by using this tunnel magnetoresistive effect. The magnitude of this resistance value becomes the maximum value when the magnetization direction of the storage layer and the magnetization direction of the magnetization fixed layer are anti-parallel to each other and it becomes the minimum value when they are parallel to each other.
According to the magnetic memory device having the above-mentioned arrangement, information can be written (recorded) on the magnetic memory device by controlling the magnetization direction of the storage layer of the magnetic memory device with application of a synthesized electric current magnetic field generated when an electric current flows through both of the word line and the bit line. It is customary to store a difference between the resultant magnetization directions (magnetized states) of the storage layer in response to “0” information and “1” information.
When on the other hand recorded information is read out from the magnetic memory device, a memory cell is selected by using a device such as a transistor and a difference between the magnetization directions of the storage layer is detected as a difference between voltage signals by using the tunnel magnetoresistive effect of the magnetic memory device, whereby recorded information can be detected.
Having compared this MRAM with other nonvolatile memories, it is to be understood that the maximum merit of the MRAM is that, since “0” information and “1” information are rewritten by inverting the magnetization direction of the storage layer formed of the ferromagnetic material, the MRAM can be rewritten at a high speed nearly infinitely (>1015 times).
However, the MRAM has to generate a relatively large electric current magnetic field to rewrite recorded information and hence an electric current of a certain large magnitude (for example, about several milliamperes (mA)) should flow through the address wirings. Therefore, it is unavoidable that power consumption of the MRAM is increased considerably.
Also, the MRAM needs write address wiring and read address wiring and hence it has been difficult to microminiaturize a memory cell from a structure standpoint.
Further, as the device is microminiaturized increasingly, the address wiring also is reduced in width so that it becomes difficult to apply a sufficient electric current to the address wiring. In addition, since coercive force of the device is increased, a necessary electric current magnetic field is increased and hence power consumption of the device is increased.
Accordingly, it has been difficult to microminiaturize the device.
For this reason, a memory having an arrangement to use magnetization inversion generated by spin transfer receives a remarkable attention as an arrangement capable of inverting the magnetization direction with application of a small electric current.
Magnetization inversion generated by spin transfer is to cause magnetization inversion to occur in other magnetic material by injecting spin-polarized electrons from the magnetic material to other magnetic material (see Cited Patent Reference 1, for example).
Specifically, magnetization inversion generated by spin transfer is a phenomenon to give torque to the magnetization of this magnetic layer when spin-polarized electrons passed through the magnetic layer (magnetization fixed layer) of which magnetization direction is fixed enter other magnetic layer (magnetization free layer) whose magnetization direction is not fixed. Then, the magnetization direction of the magnetic layer (magnetization free layer) can be inverted with application of an electric current of a magnitude higher than a certain threshold value.
For example, with application of an electric current to a giant magnetoresistive effect device (GMR device) or a magnetic tunnel junction device (MTJ device) including a magnetization fixed layer and a magnetization free layer in the direction perpendicular to the film plane thereof, the magnetization direction of at least a part of the magnetic layer of these device can be inverted.
Thus, when the storage element including the magnetization fixed layer and the magnetization free layer (storage layer) is constructed and a polarity of an electric current flowing through the storage element is changed, the magnetization direction of the storage layer is inverted to rewrite “0” information and “1” information.
When recorded information is read out from the memory, recorded information can be read out from the memory by using the tunnel magnetoresistive effect similarly to the MRAM because this memory has the arrangement in which the tunnel insulating layer is provided between the magnetization fixed layer and the magnetization free layer (storage layer).
Then, magnetization inversion based on spin transfer has an advantage in that magnetization inversion can be realized without increasing an electric current even when the device is microminiaturized.
An absolute value of an electric current flowing through the storage element to invert the magnetization direction is less than 1 mA in a storage element of the scale of approximately 0.1 μm, for example. In addition, the above-mentioned absolute value is decreased in proportion to a volume of a storage element, which is advantageous from a scaling standpoint.
In addition, since the recording word line, which has been required by the MRAM, becomes unnecessary, this memory has an advantage in that the arrangement of the memory cell can be simplified.
[Cited Non-Patent Reference 1]: NIKKEI ELECTRONICS, 2001, VOL. 2. 12 (pp. 164 to 171)
[Cited Non-Patent Reference 2]: J. NaHas et al., IEEE/ISSCC 2004 Visulas Supplement, p. 22
[Cited Patent Reference 1]: Official Gazette of Japanese laid-open patent application No. 2003-17782
FIG. 1 of the accompanying drawings is a schematic cross-sectional view showing an arrangement of a storage element capable of recording information by using spin transfer according to the related art.
As shown in FIG. 1, this storage element 110 is composed of an underlayer 101, an antiferromagnetic layer 102, a magnetization fixed layer 103, a non-magnetic layer 104, a storage layer 105 and a capping layer 106 laminated with each other, in that order, from the lower layer.
The storage layer 105 is made of a ferromagnetic material having uniaxial magnetic anisotropy and the storage element 110 is able to store information therein depending on the magnetization state of this storage layer 105, that is, the direction of a magnetization M112 of the storage layer 105.
The magnetization fixed layer 103 made of a ferromagnetic material and of which direction of a magnetization M111 is fixed is provided through the non-magnetic layer 104 to the storage layer 105. In the arrangement shown in FIG. 1, since the antiferromagnetic layer 102 is formed on the lower layer of the magnetization fixed layer 103, the direction of the magnetization M111 of the magnetization fixed layer 103 is fixed by the action of this antiferromagnetic layer 102.
When information is written in this storage element 110, the direction of the magnetization direction M112 of the storage layer 105 is inverted based on spin transfer with application of an electric current flowing through the direction perpendicular to the film plane of the storage layer 105, that is, the lamination layer direction of the storage element 110.
Magnetization inversion based on spin transfer will be described in brief.
Electron has two kinds of spin angular momentums. Let it be assumed hat the two kinds of spin angular momentums are defined as upward spin angular momentum and downward spin angular momentum. Both of the upward spin angular momentums and the downward spin angular momentums are of the same number within the non-magnetic material but they are different in number within the ferromagnetic material.
In the storage element 110 shown in FIG. 1, let it be considered the case in which the directions of the magnetic moments are anti-parallel to each other in the magnetization fixed layer 103 and the storage layer 105 and in which electrons are to be transferred from the magnetization fixed layer 103 to the storage layer 105.
Electrons passed through the magnetization fixed layer 103 are spin-polarized so that the upward spin angular momentum and the downward spin angular momentum are different from each other in number.
If electrons reach the other magnetic material before the thickness of the non-magnetic layer 104 is sufficiently thin so that spin polarization is relaxed and electrons are placed in the non-polarized state (upward spin momentum and downward spin momentum are the same in number) of the ordinary non-magnetic material, then since the directions of the magnetic moments of the magnetization fixed layer 103 and the storage layer 1–5 are anti-parallel to each other and signs of degree of spin polarization are opposite to each other, a part of electrons is inverted, that is, the direction of the spin angular momentum is changed in order to decrease energy of the system. At that time, since a total angular momentum of the system should be preserved, reaction equivalent to the total of angular momentums changed by electrons of which directions are changed is given to the magnetic moment of the storage layer 105.
When there are few electric currents, that is, electrons passed at the unit time, there are a small total number of electrons whose directions are to be changed so that the change of the angular momentum generated in the magnetic moment of the storage layer 105 is small. However, when an electric current is increased, many changes of the angular momentums can be given to the electrons within the unit time. The time change of the angular momentum is torque. When torque exceeds a threshold value, the magnetic moment M112 of the storage layer 105 starts to be inverted and it is stabilized after it was rotated 180 degrees owing to its uniaxial magnetic anisotropy. That is, the magnetic moment is inverted from the anti-parallel state to the parallel state.
On the other hand, when the directions of the magnetic moments are parallel to each other in the magnetization fixed layer 103 and the storage layer 105, if an electric current flows through the direction to transfer electrons from the storage layer 105 to the magnetization fixed layer 103, then torque is applied to the magnetization fixed layer 103 and the storage layer 105 when electrons spin-inverted after they were reflected on the magnetization fixed layer 103 enter the storage layer 103 with the result that the magnetic moments can be inverted from the parallel state into the anti-parallel state.
However, an amount of an electric current required to invert the magnetic moments from the parallel state to the anti-parallel state is increased more as compared with that required when the magnetic moments are inverted from the anti-parallel state to the parallel state.
As described above, information (“0” information and “1” information) is recorded on the storage layer 105 with application of electric currents higher than a certain threshold value corresponding to the respective polarities in the direction from the magnetization fixed layer 103 to the storage layer 105 and vice versa.
Also, information can be read out from the storage layer 105 by using a resistance change dependent on a relative angle between the magnetic moments of the storage layer 105 and the magnetization fixed layer (reference layer) 103, that is, a so-called magnetoresistive effect in which the minimum resistance is obtained when the magnetic moments are parallel to each other and in which the maximum resistance is obtained when the magnetic moments are anti-parallel to each other.
Specifically, when a substantially constant voltage is applied to the storage element 110 and a magnitude of an electric current flowing at that time is detected, information can be read out from the storage layer 105.
In the following description, a relationship between the resistance state of the storage element 110 and information will be prescribed in such a manner that a low resistance state is prescribed as “1” information, a high resistance state being prescribed as “0” information, respectively.
Also, an electric current to transfer electrons from the capping layer 106 shown in FIG. 1 to the underlayer 101, that is, from the upper layer to the lower layer is prescribed as a positive polarity electric current. At that time, when a positive polarity electric current flows through the storage element 110, electrons are transferred from the capping layer 106 to the underlayer 101, that is, from the storage layer 1–5 to the magnetization fixed layer 103 so that the direction of the magnetization M111 of the magnetization fixed layer 103 and the direction of the magnetization M112 of the storage layer 105 are placed in the anti-parallel state, thereby resulting in the storage element 110 being set to the high resistance state as mentioned hereinbefore.
Accordingly, an electric current to write “1” information (low resistance state) becomes negative in polarity and an electric current to write “0” information (high resistance state) becomes positive in polarity.
The polarity of electric current to write information, that is, whether a negative polarity write electric current −Iw corresponds to writing of “1” information or writing of “0” information is prescribed based on the arrangement of the multilayer film of the storage element and the like.
FIG. 2 is a schematic diagram showing a mutual relationship between operation electric currents for write operation and read operation in a storage element like the storage element 110, shown in FIG. 1, for recording information by using spin transfer. In FIG. 2, a horizontal axis represents an electric current and a vertical axis represents the number of elements through which an electric current of a certain magnitude flows upon predetermined operation.
FIG. 2 shows distributions and average values of electric currents +Iw and −Iw which are required to write “1” information or “0” information, respectively.
Further, in FIG. 2, Ir0 and Ir1 denote distributions and average values of electric currents required upon reading. Ir0 with a small electric current amount corresponds to the reading of the high resistance state (“0” information) and Ir1 with a large electric current amount corresponds to the reading of the low resistance state (“1” information). In FIG. 2, reference numeral Ic denotes an electric current which flows through a read reference cell (cell to generate a reference electric current for use with an operational amplifier for comparison). Reference numeral ΔI denotes a difference current corresponding to the change of resistance upon reading.
As is clear from FIG. 2, for example, it is customary that absolute values of the electric currents −Iw and +Iw required to write “1” information and “0” information are not equal to each other. As shown in FIG. 2, the absolute value of the positive polarity electric current +Iw is larger than that of the negative polarity electric current −Iw.
Asymmetric property of the write electric currents −Iw and +Iw of both polarities is generated not only based on the recording principle of the above-mentioned spin transfer but also influences of the external magnetic field.
FIG. 3 is a diagram showing influences exerted upon the write electric currents by the external magnetic field. In FIG. 3, a horizontal axis represents an external magnetic field Hext normalized by a magnetic anisotropy Hk of the storage layer and a vertical axis represents a write electric current Ic similarly normalized.
A study of FIG. 3 reveals that, if the absolute values of the write electric currents of both positive and negative polarities are equal to each other without application of the external magnetic field (Hext=0), then when the external magnetic field Hext is applied, the absolute values of the write electric currents of both positive and negative polarities become different from each other.
Then, if the memory is constructed by using the magnetization inversion based on the above-mentioned spin transfer, when respective information (“0” information and “1” information) are recorded on the storage layer, polarities of electric currents applied to the storage element should be changed as in the positive polarity +Iw and the negative polarity −Iw, respectively.
For this reason, there is employed a method in which transfer gates composed of P type and N type MOS transistors are used as memory cell selection switches for the storage element in each memory cell.
However, even when any method is employed, it has been difficult to make the absolute values of maximum saturation electric currents, which can be applied with respective polarities in both directions, become equal to each other.
As a result, in order to guarantee a saturation electric current of one polarity sufficient to record information, the size of the transistor is increased unavoidably and hence this hinders the memory from becoming high in density.